1. Field of Invention
This disclosure relates generally to a method and arrangement for determining a sample flow volume.
2. Description of the Prior Art
Breath gases are measured in hospitals for several reasons and several departments. Typical measured gases are one or several of the following gases: CO2, O2, N2O and anesthetic gases (halothane, sevoflurane, desflurane, isoflurane and enflurane). CO2 measurement can also be used to determine respiration rate. Gases are measured either using a sidestream (non-diverting) technique or a mainstream (diverting) technique. The sidestream technique means that the gas sample is transferred along a sampling line from a gas channel, which is between a patient and a ventilator, to a sensor, which causes a delay between breath and measurement. Mainstream technique means that sample is analyzed at sampling site, which is typically in the gas channel between the patient and the ventilator.
Spirometry measurement is used to measure lung function by measuring the flow and pressure of the inhaled and exhaled air. The volume of the breath can be calculated of those parameters.
By combining the breath gas measurement and the spirometry measurement a gas exchange measurement can be performed. It means that by synchronizing gas measurement concentration curves of carbon dioxide (CO2) and oxygen (O2) and spirometry flows and pressures, the oxygen consumption (VO2) and produced carbon dioxide (VCO2) can be measured. These parameters reflect the metabolic component of body systems and can be used to further calculate a respiratory quotient (RQ) and an energy expenditure (EE). The continuous and non-invasive measurement of respiratory gas exchange, also known as indirect calorimetry, is potentially valuable when employed for diagnostic and therapeutic purposes.
The sidestream measurement includes a delay, which can be from one to several seconds depending on the sampling line length, sample line inner diameter and sample flow rate. The spirometry measurement does not include this delay and therefore the right synchronization of these measurements is crucial for the gas exchange measurement. There is no good and reliable method to identify which spirometry breath cycle belongs to which gas measurement cycle. This problem in synchronization is the bigger the higher is the respiration rate. The delay between these measurements may be even a few breathing cycles long.
The synchronization would be quite easy if there was only one sample line length and only one sample line inner diameter available because then the sampling volume, which is the volume between the end of sampling line, which is a point of contact where the sampling line is connected to the gas channel, and gas sensor, would be known. The synchronization is performed by integrating the sampling flow rate acquired by the internal flow sensor in the gas measuring unit starting for example from a beginning of an inspiration and integrating as long as the sampling line volume is reached and selecting the nearest beginning of the inspiration from the gas sensor. If there is one beginning of the inspiration much nearer than the second nearest, then the synchronization was successful. Usually there are, however, many lengths of sampling lines and even many sample line inner diameters available and therefore that technique can't be used or it is unreliable.
The synchronization can be performed also utilizing the previous technique if there are for example two sample line lengths and one sample line inner diameter or one sample line length and two sample line inner diameters available but then the successfulness of the synchronization depends on whether breathing rate is suitable. At high breathing rates this technique usually can't be used because the time between beginning of breathing cycles is short and uncertainty, which breathing cycle is the right one, is too big.
The sampling line volume and sampling delay can be determined also by stopping a pump withdrawing the gas from the gas channel to the sidestream gas sensor and waiting for some breathing cycles. There will be mixed air in the end of the sampling line which is close to the gas channel. By starting the pump and calculating the time and sampling line volume till the mixed air receives the gas sensor, the sample volume and delay time can be determined and the synchronization usually is successful. The problem is that this technique forces to stop the pump so the gas measurement is not available for many seconds.
A present well-known theoretical background of a technique for synchronization is based on an adjustment of the sample flow rate. The sampling volume Vsample is an integral of the sample flow rate integrated for delay time, Tdelay:
      Vsample    =                  ∫        0        Tdelay            ⁢              F        ⁢                  ⅆ          t                      ,where F=sample flow rate. The delay is not known, so the sampling volume can't be calculated. If two different sample flow rates are used, then:
      Vsample    =                            ∫          0                      Tdelay            ⁢                                                  ⁢            1                          ⁢                  F          ⁢                                          ⁢          1          ⁢                      ⅆ            t                              =                        ∫          0                      Tdelay            ⁢                                                  ⁢            2                          ⁢                  F          ⁢                                          ⁢          2          ⁢                      ⅆ            t                                ,where F1 is a sample flow rate1, Tdelay1 is a respective delay and F2 is the a sample flow rate2 and Tdelay2 is a respective delay. By changing the sample flow rate, the delay changes. If the sample flow rate is averaged during the delay time (=F1ave) in the following way:
            F      ⁢                          ⁢      1      ⁢      ave        =                  ∫        0                  Tdelay          ⁢                                          ⁢          1                    ⁢              F        ⁢                                  ⁢        1        ⁢                              ⅆ            t                    /                      delay            ⁢            1                                ,
Vsample is then:Vsample=F1ave*Tdelay1=F2ave*Tdelay2
If the difference between delays is ΔTdelay, thenVsample=F1ave*Tdelay1=F2ave*(Tdelay1+ΔTdelay).
Delay1 can be solved from the equation:Tdelay1=F2ave*ΔTdelay/(F1ave−F2ave).
The sampling volume can now be expressed with ΔTdelay and flows:Vsample=F1ave*Tdelay1=F1ave*F2ave*ΔTdelay/(F1ave−F2ave).
If units such as ml/min as flow unit, s as Tdelay unit and ml as volume unit are used, thenVsample=(F1ave/60)*Tdelay1=F1ave*F2ave*ΔTdelay/(60*(F1ave−F2ave)).
The delay can now be calculated:Tdelay=60*Vsample/Fave,where Fave is the average sample flow.
Various other features, objects, and advantages of the invention will be made apparent to those skilled in art from the accompanying drawings and detailed description thereof.